In recent years, solid state light emitting elements, such as light emitting diodes (LEDs), have become available that produce relatively high intensities of output light, which has enabled use of such devices in general light applications, signage applications and a verity of other areas long dominated by more conventional light sources, such as incandescent and florescent lights. Many lighting applications require control of the level of output over a fairly wide range of light output, that is to say from a minimum light output level up to some full-ON output level. Most conventional light sources can be ‘dimmed’ in a relatively continuous manner, e.g. by adjusting the level or duty cycle of the voltage/current applied to drive the light source. Systems using solid state light emitting elements also can have intensity control, for dimming or the like, and there have been several ways to implement such output level control.
One approach to controlling the light output of a system or device that uses one or more solid state light emitting elements involves adjusting the magnitude of the current applied to drive the solid state light emitting element(s). LEDs, for example, produce a light output that is roughly proportional to the applied current level, over some portion of their operating range. However, for substantial changes, the changes in operating state of a LED will change the color of the light output. For example, if a LED is rated to output a particular color of light for a specified input drive current, the output light color will shift from the rated color as the input drive current is reduced substantially below the specified drive current level. Hence, dimming light output of solid state light emitting elements often is limited by the degree of current level reduction possible before there is an undesirable degree of color shift.
Another approach to controlling the light output of a system or device that uses one or more solid state light emitting elements involves modulating the signal used to drive the solid state light emitting element(s) and varying the degree of modulation. Amplitude modulation may cause a color shift, similar to that resulting from reduction in the magnitude of the drive current. Hence, many systems today control light output of LEDs or other solid state emitters by pulse width or duty cycle type modulation. With a pulse width modulation (PWM) approach for an LED based system, for example, each LED is driven by a pulse signal. The frequency of the pulse signal is sufficiently high that a human observer normally does not perceive the pulse of the LED output. Adjustment of the percent modulation of the pulse drive signal adjusts the width of the drive pulse, and thus the ON-time of the LED during each cycle of the pulse signal. The human eye tends to perceive an integral of light over short periods of time, so the adjustment of the LED ON-time, adjusts the perceived light output of the LED. With a PWM approach, however, if the ON-time is particularly low (for a desired low output level) the human observer may see a perceptible flicker.
Solid state lighting emitting elements provide relatively small point source outputs. For many lighting applications, each lighting system includes a number of LEDs or other solid state lighting emitting elements. Often, the system will include a diffuser or the like. However, within a certain range of distance from the system, an observer can see the individual outputs of the solid state lighting emitting elements. In many applications, the individual point source appearance may be undesirable. Not all emitters respond identically to a given input signal, particularly over a wide range of variations of the input signal. Variations in drive signals applied to different LEDs or the like to change system output level may cause different solid state lighting emitting elements within one system to provide different levels or colors of light output. In system arrangements where the point source outputs are perceptible, the differences in output level or color also will be perceptible to the human observer. For example, if an individual LED in an array cuts-OFF, because of a low drive current level, but other LEDs in the array remain ON, the observer will see a black spot or hole in the array at the location of the LED that is not emitting any light.
A need exists for a way in which to control light output level, in solid state lighting systems for a variety of different types of lighting applications, which avoids or mitigates any or all of the above discussed problems.